diff --git a/litellm/utils.py b/litellm/utils.py
index a7d7447d7f3..4a51336a862 100644
--- a/litellm/utils.py
+++ b/litellm/utils.py
@@ -4517,6 +4517,15 @@ def get_optional_params(
                 unsupported_params[k] = non_default_params[k]
 
         if unsupported_params:
+            if "reasoning_effort" in unsupported_params and custom_llm_provider in ("openai", "azure"):
+                raise UnsupportedParamsError(
+                    status_code=500,
+                    message=(
+                        f"{custom_llm_provider} does not support reasoning_effort for model={model}. "
+                        "reasoning_effort cannot be silently dropped. To forward it to the provider, "
+                        "send allowed_openai_params=['reasoning_effort'] in your request."
+                    ),
+                )
             if litellm.drop_params is True or (drop_params is not None and drop_params is True):
                 for k in unsupported_params:
                     non_default_params.pop(k, None)
diff --git a/tests/unit/test_utils.py b/tests/unit/test_utils.py
index a72aec07766..a0a7eff505e 100644
--- a/tests/unit/test_utils.py
+++ b/tests/unit/test_utils.py
@@ -6144,6 +6144,42 @@ class TestFinalOptionalParamsLineRedaction:
         assert "'temperature': 0.25" in printed
 
 
+@pytest.mark.parametrize(
+    "custom_llm_provider, model",
+    [
+        ("openai", "gpt-4.1"),
+        ("openai", "gpt-4.1-mini"),
+        ("openai", "gpt-4o"),
+        ("openai", "gpt-4o-mini"),
+        ("azure", "gpt-4.1"),
+        ("azure", "gpt-4.1-mini"),
+        ("azure", "gpt-4o"),
+        ("azure", "gpt-4o-mini"),
+        ("azure", "azure/eu/gpt-4o-2024-08-06"),
+    ],
+)
+@pytest.mark.parametrize("request_drop, global_drop", [(True, False), (None, True)])
+def test_unsupported_reasoning_effort_is_not_silently_dropped(
+    custom_llm_provider: str,
+    model: str,
+    request_drop: bool | None,
+    global_drop: bool,
+    monkeypatch: pytest.MonkeyPatch,
+) -> None:
+    from litellm.utils import get_optional_params
+
+    # https://github.com/BerriAI/litellm/issues/40470, verified 2026-10-02.
+    # https://github.com/BerriAI/litellm/pull/44060#discussion_r4159472144, verified 2026-10-02.
+    monkeypatch.setattr(litellm, "drop_params", global_drop)
+    with pytest.raises(litellm.UnsupportedParamsError, match="reasoning_effort"):
+        get_optional_params(
+            model=model,
+            custom_llm_provider=custom_llm_provider,
+            reasoning_effort="low",
+            drop_params=request_drop,
+        )
+
+
 class TestDropParamsStringCoercion:
     @pytest.mark.parametrize("drop_params", ["true", "True", True])
     def test_truthy_drop_params_drops_unsupported_temperature(self, drop_params, monkeypatch):